The present invention relates in general to communication systems, and is particularly directed to the use of demultiplexed quarter-rate, integrated services digital network (ISDN) channels, to extend the operating range of four-wire, Dataphone Digital Service (DDS) subscriber lines to distances (on the order of 35 kft) well beyond distances currently possible at T-carrier time slot channel data rates (56 kbps and 64 kbps).
Co-pending U.S. patent application Ser. No. 08/500,441, filed Jul. 10, 1995, now U.S. Pat. No. 5,598,413 issued Jan. 28, 1997, entitled: xe2x80x9cFOUR WIRE, HALF-RATE ARCHITECTURE WITH EMBEDDED DIFFERENTIAL DELAY COMPENSATION FOR EXTENDING RANGE OF BASIC RATE ISDN COMMUNICATIONS,xe2x80x9d by M. Sansom et al, assigned to the assignee of the present application and the disclosure of which is herein incorporated (hereinafter referred to as the ""441 application), describes a one-half rate ISDN demultiplexingxe2x80x94multiplexing architecture for solving a communication range extension problem resulting from an American National Standards Institute (ANSI) standard T1.601 governing ISDN communications.
In particular, ANSI standard T1.601 for 2B1Q modulation, two-wire, full-duplex data transfer with echo cancellation requires that currently installed ISDN basic rate digital subscriber lines (having a data rate of 144 kbps, with bidirectional data payload, plus overhead maintenance channels) must not exceed a two-wire loop loss of 42 dB at 40 kHz, or 1300 ohms, resistive. Such a requirement effectively limits the operational range of such a two-wire loop to approximately 15.2 kft, using No. 26 (American Wire Gauge) wire, and commercially available ISDN transceiver equipment. Extending ISDN communications to customers geographically located beyond this range requires that the service provider either install repeaters in the loop, or use a different communication medium, such as a T1 carrier fiber optic link, with both solutions being unattractive from a standpoint of cost, installation and maintenance.
In accordance with the invention described in the ""441 application, this problem is solved by an ISDN communications architecture employing a pair of demultiplexed half-rate, integrated services digital network (ISDN) channels, having an out-of-band maintenance channel for conveying differential delay compensation information, to extend the normal range of ISDN basic rate digital subscriber lines to distances (on the order of 25 kft) well beyond those currently possible (typically 15.2 kft) using a repeaterless two-wire transmission path.
A problem similar to that described above for ISDN basic rate digital subscriber lines is encountered in (four-wire) DDS communications. In particular, if a telecommunications loop is to be a candidate for DDS assignment, it must comply with ATandT Technical Reference Publication No. 62310, which requires that the measured insertion loss in one direction between an Office Channel Unit Data Port (OCU DP) and the customer premises point of demarcation be less than 34 dB, when measured at the Nyquist frequency (xc2xd the data rate for alternate mark inversion (AMI) circuits) and terminated by a 135 ohm impedance. This equates to approximately 21 kft of 24 AWG (American Wire Gauge) copper wire, or 18 kft of mixed gauge 26 and 24 AWG copper wire.
Although many OCU DP units can accommodate 45 dB of signal loss over copper wire loops, the requirement for the customer-owned equipment in terms of receiver sensitivity is only 38 dB, so that extending DDS communications to customers premises located beyond the 34 dB range requires that the service provider install either a loop repeater or a T1 Digital Loop Carrier system, each of which entails a substantial cost and installation penalty, as described above.
DDS loop repeaters are currently deployed on circuits operating at data rates of 56 kbps, 56 kbps with secondary channel, and 64 kbps. The effective communication range of circuits operating at data rates below 56 kbps can be extended to greater distances and therefore seldom require the use of a DDS repeater. A DDS repeater is powered from the loop and requires specially equipped OCU DP units or additional line powering modules, as well as a repeater housing or environmentally hardened case that can be mounted on a pole or within a subterranean enclosure.
In addition to the expense and time associated with the installation of the repeater and associated components is the expense incurred in the course of periodic maintenance of the repeater enclosure. While Digital Loop Carrier (DLC) systems offer a favorable solution to the dilemmas posed in DDS range extension when available, their cost cannot be justified on an individual circuit basis, due to the considerable investment for the service provider in terms of cost and installation time.
In accordance with the present invention, the desire to extend DDS communications to customer premises beyond the presently allowable four-wire loop range of approximately 18 kft (56 kbps, 56 kbps with secondary channel capability, and 64 kbps) using a mixed 24 and 26 AWG copper wire loop, without the cost penalty of the alternatives described above, is successfully addressed by employing commercially available ISDN transceiver chip hardware to demultiplex quarter-rate (2B1Q) ISDN channels for transport of DDS data over the four-wire DDS transmission path between the OCU DP and a customer premises site.
By operating at a frequency that is one-quarter of the operating frequency associated with the ISDN transceivers, the reduced data rate of the four-wire system operates as a trade-off against loop loss, increasing the distance over which DDS may be provided without further stipulation or constraint upon the requirement of loops considered as DDS candidates in terms of loop loss and bridged tap.
In accordance with a preferred embodiment of the DDS architecture of the present invention, an xe2x80x98upstreamxe2x80x99 OCU DP site is operative to demultiplex PCM data associated with alternate successive pairs of data bytes associated with a respective timeslot of a T-carrier system into two ISDN transceiver units, each of which is clocked at one-quarter the rate of basic ISDN signalling of 80 kHz. Quarter-rate ISDN bearer (B) channels of DDS data are transported over two, two-wire pairs that make up the four-wire transmission path to a pair of ISDN transceiver units at the xe2x80x98downstreamxe2x80x99 termination unit installed at a customer premises site. The downstream termination unit multiplexes these quarter-rate ISDN channels back into a single data stream which is delivered to the customer as an alternate mark inversion (AMI) modulated signal.
Because the quarter-rate ISDN channels are demultiplexed onto separate physical two-wire communication paths, there will be a differential transport delay or offset between the two pairs of quarter-rate ISDN channels, which must be corrected to gain proper time alignment and ensure that data is multiplexed back into the original data stream.
This differential transport delay between the two pairs of quarter-rate ISDN channels is compensated by using an available overhead or auxiliary channel of an unused quarter-rate (4 Kb/s) signalling (D) channel or an out-of-band (4 Kb/s) maintenance channel to convey time of start and time of arrival measurement information to the opposite end of the four-wire link, and thereby enable supervisory communication control processors at one or both ends of the four-wire link to control the insertion of the requisite amount of delay in the faster of the two pairs of quarter-rate ISDN channels, achieving the necessary time alignment for multiplexing the quarter-rate ISDN channels back into a single data stream prior to AMI conversion for delivery to customer premises equipment.
The invention takes advantage of the communication signal processing functionality of the ISDN transceiver chips to pre-establish, at the OCU DP end of the four-wire link, a prescribed differential (lead time) offset between the superframe synchronization signal structure inherent in the operation of each transceiver chip for a first (or leading) of one pair of quarter-rate ISDN channels relative to the superframe synchronization signal structure for a second (or lagging) pair of quarter-rate ISDN channels.
In addition, an available (normally unused) portion of the out-of-band maintenance channel of the leading quarter-rate ISDN channel or signalling (D) channel may used to mark that channel as having its leading superframe synchronization structure started first, so that transceiver equipment at the downstream DDS termination unit will know which received pair of the quarter-rate ISDN channels contains a xe2x80x98leadingxe2x80x99 channel, regardless of which two-wire pair has conveyed that quarter-rate ISDN channel. This xe2x80x98markingxe2x80x99 of such a leading channel allows the connections of the two twisted pairs to the transceiver units at the ends of the four-wire link to be physically reversed, without impairing the signal processing capabilities of the system.
At the downstream (termination unit) end of the four-wire link, the differential delay between the superframe boundaries of the two (leading and lagging) pairs of quarter-rate ISDN channels is measured and compared with the expected delay offset between the marked (leading) quarter-rate channel of one pair of ISDN channels, and an unmarked (lagging) quarter-rate ISDN channel of the other pair of ISDN channels. Knowing that the superframe synchronization signal structure of the marked (leading) quarter-rate ISDN channel was started first by its quarter-rate clocked transceiver at the OCU DP, and by a prescribed lead time relative to the lagging quarter-rate ISDN channels of the other pair, the difference in transport delay between the two pairs of quarter-rate ISDN channels is computed by the customer premises-associated DDS termination unit""s communications control processor.
In accordance with a first embodiment of the invention, this computed delay is employed by the customer premises-associated DDS termination unit""s processor to control the injection of the necessary delay offset (by way of delay circuitry contained in the multiplexer for combining the two pairs of received quarter-rate ISDN channels) into the faster pair of channels, thereby providing the required time alignment for recovering the DDS data stream.
In a second embodiment of the invention, rather than provide delay equalization in the multiplexer path within the DDS termination unit, the DDS termination unit""s processor injects a received delay representative message into a signalling (D) channel portion or out-of-band embedded operations channel (EOC) or maintenance channel portion of one of the quarter-rate ISDN channels in the return path (from the DDS termination unit back to the OCU DP). This delay offset message in the return channel is used by the OCU DP""s communications control processor to control the injection of the insertion of a prescribed delay into the faster pair of quarter-rate ISDN channels demultiplexed at the OCU DP, thereby providing pre-transmission time alignment that will allow the multiplexer in the downstream DDS termination unit to recover the data stream. Thus, signal processing hardware at the DDS termination unit is simplified by performing all delay equalization at the OCU DP.
For the quarter-rate ISDN channels in the return direction, since the superframes are defined by the downstream superframe boundaries inherent in the received quarter-rate ISDN channels, it is not possible to use the quarter-rate transceiver chips in the DDS termination unit to invoke a prescribed (leading and lagging) offset between the superframe structures of the respective return quarter-rate channels as carried out in the OCU DP, as described above.
This is circumvented by measuring the actual offset between the superframe boundaries of the return channels, and then incorporating a return channel framing message representative of the relative delay between the framing structures of the return quarter-rate channels into a usurped portion of an out-of-band maintenance channel or signalling (D) channel portion of one of the quarter-rate channels in the return path, similar to injection of the received delay representative message in a signalling or out-of-band maintenance channel portion of the one quarter-rate return channel.
At the OCU DP, similar to the framing delay measurement processing carried out at the DDS termination unit, the differential delay between the superframe structures of the two pairs of return quarter-rate ISDN channels is measured and compared with the measured framing transmission offset message in the out-of-band maintenance channel portion or the unused signalling channel portion of the one return path quarter-rate channel. Any difference between the two is employed to delay the faster pair of quarter-rate ISDN channels within the multiplexed digital signal path at the OCU DP, thereby correcting for the differential transmission path delay in the return direction.